The present invention relates generally to optical projection systems, and more particularly to off-axis catadioptric (including lenses and mirrors) reduction systems such as might be useful for microlithography.
As the feature sizes on integrated circuit devices have grown ever smaller, the demands on the optical train in the microlithography system have grown ever greater. In order to achieve resolution characteristics that are commensurate with future requirements for optical systems, the numerical aperture required is in excess of the current state-of-the-art optical systems. For resolution less than 0.35 .mu.m, a numerical aperture in excess of 0.50 is needed. The prior art, as exemplified by U.S. Pat. Nos. 4,685,777 and 4,701,035, describes reducing optical projection systems where the bulk of the optical power is developed with curved mirrors, and lenses are used to correct aberrations. The systems described in these patents have numerical apertures of 0.25 and 0.18 respectively.
In order to achieve a degree of compactness, the optical path is often folded, either by the curved mirrors or by flat folding mirrors introduced into the optical path. A number of the prior art systems incur the need to use truncated and in some cases decentered optical elements in order to avoid obscuring the path of the beam as it is reflected back. A problem with using truncated lens elements is that the elements are much more difficult to align along the optical axis, and aberrations and errors introduced by decentration and the like are significant. In the manufacturing process the requirement for maintaining contour accuracy of the optical surfaces increases the cost of the components considerably relative to that of non-truncated equivalents. A further problem with truncated optical elements occurs in the application of the system when, due to the use of a high-powered laser, optical elements are subject to increased temperatures which causes non-radially-symmetric gradients in the truncated components. This could reduce the performance of systems containing those elements considerably.
The various design problems become more challenging as the need for systems with higher numerical aperture becomes greater. Larger reflecting elements are required, adding cost and size. Moreover, there appears to be a conflict between large numerical aperture and adequate working distance (back focal length). It is also typically difficult to find space for field stop baffles or an aperture stop.